The present invention pertains to replaceable cartridge type filters and, more particularly, to such a filter utilizing a semipermeable membrane cartridge with an assembly for retaining the cartridge in the housing when it is removed for replacement.
Replaceable cartridge filters have been used in water treatment systems for many years. A typical filter unit of this type utilizes an elongate, generally cylindrical housing or sump which is open at one end for the receipt of a replaceable filter cartridge. The cartridge and the housing are demountably attachable to an enclosing end cap. The end cap may be a unitary cover or part of a header or manifold system for control of fluid flow into and out of the cartridge. The filter cartridge may include any of several well known kinds of filter media, and multiple housings and associated filter cartridges may be attached to a common header system for the serial removal of a wide variety of both suspended and dissolved contaminants from a liquid stream. The media used in filter cartridges varies widely, depending upon the contaminant to be removed, and such media include granular particulate materials, coarse sintered blocks, paper and plastic filters, and semi-permeable membranes.
One common type of reverse osmosis (RO) filter cartridge utilizes a membrane which is spirally wrapped around a porous center tube and enclosed in an impervious cylindrical outer wrap. The opposite ends of the cartridge are open, with one of the ends receiving untreated incoming water under pressure and in which most dissolved solids are separated as the water passes through the semipermeable membrane and flows radially toward the porous center tube. The treated water (or membrane permeate) passes along the center tube to an outlet end. The untreated water containing the concentrated dissolved solids (known as the concentrate and commonly referred to as brine) passes out of the opposite end of the membrane cartridge. Such a cartridge typically includes a brine seal which is interposed between the outer cartridge wrap and the inside wall of the cartridge housing to prevent untreated water from bypassing the cartridge and mixing directly with the brine. However, in certain RO cartridge assemblies, such as an assembly in which the brine seal is located at one end of the element and the outlet for the brine flow is located at the opposite end of the element and housing, a volume of stagnant water is created between the outer diameter (xe2x80x9cODxe2x80x9d) of the element and the inner diameter (xe2x80x9cIDxe2x80x9d) of the housing. This pool of stagnant water is not flushed by continuous flow through the element and therefore provides an area for the propagation of bacteria which, in turn, may lead to fouling by migration to active areas of the RO membrane surface. Depending on how a system is designed, the potential problem of pools of stagnant water, either on the untreated water side or the brine water side of the RO element, is a problem in many RO systems which utilize a brine seal.
The product water outlet in the typical RO filter cartridge comprises a cylindrical neck, which is typically an extension of the center product water tube, and is received in a cylindrical sleeve which forms part of the housing end cap and contains a product water discharge port. The interface between cartridge neck and the sleeve on the cap must be tightly sealed and one typical seal arrangement comprises a pair of axially spaced O-rings which are seated in annular grooves in the cartridge neck and which engage the inside wall of the sleeve in the cap. RO filter cartridges of the foregoing general type are shown, for example, in U.S. Pat. Nos. 4,645,601, 5,002,664, 5,082,557, 5,266,195 and 5,389,260.
A typical application for a reverse osmosis membrane filter cartridge of the foregoing general types is in purifying tap water for drinking. As indicated, such a filter cartridge is commonly utilized in a series arrangement with other replaceable cartridge filters which remove from the untreated water other suspended and dissolved solids which cannot be removed by membrane separation. Such water treatment units are commonly mounted beneath a sink on which the tap is located or in another confined and typically somewhat restricted space. A number of problems relating to the replacement of filter cartridges generally and RO filter cartridges particularly have arisen with the use of these systems.
In multi-cartridge systems which may include three units, manufacturers like to utilize identical filter housings for simplicity in manufacturing and inventory, as well as to maintain a uniform product appearance. However, certain filter cartridges and often RO membrane cartridges, do not fit well in housings designed for other types of filter cartridges used in these multi-cartridge systems. As a result, special adapters, special covers, or special non-standard housings may have to be used. A specialized and relatively complex housing end cap is shown, for example, in the above identified U.S. Pat. No. 5,082,557. A specialized, non-standard housing is shown in U.S. Pat. No. 5,266,195.
Because RO filter cartridges must handle and provide an interface for three liquid flows, namely, untreated water, treated water (membrane permeate), and brine (membrane concentrate), some means for accommodating the additional liquid flow, not present in other kinds of filter cartridges, must be utilized. In U.S. Pat. No. 5,002,664, the brine flow is accommodated by a special connection through the bottom of the RO cartridge housing. This complicates the construction of the unit, as well as the ease of filter replacement.
Another problem unique to filter cartridges having a neck on one end for the treated water outlet, which includes an O-ring seat arrangement engaging the cylindrical ID of a sleeve in the end cap, is that the resulting tight seal causes the filter cartridge to hang up in the end cap when the housing is unscrewed from the end cap for removal and replacement of the filter cartridge. The filter housing must be slid downwardly along the entire length of the file cartridge before the cartridge can be independently removed from its sealed attachment to the end cap or header. When operating in a confined space, such as under a kitchen sink, this lack of head space may present a serious obstacle to ease of replacement. This problem is addressed and solved in U.S. Pat. No. 4,645,601 by providing an integral cartridge and housing which are removed together. Such an arrangement, though convenient, is extremely uneconomical because the housing must be replaced each time the filter cartridge is replaced.
One solution to the problem is shown in U.S. Pat. No. 5,891,334 where a cartridge retaining ring is used to hold the filter cartridge in place within the housing and to retain it in place when the housing is unscrewed or otherwise removed from the end cap or header for cartridge replacement. After insertion of the filter element into the housing, the cartridge retaining ring is placed over the upper end of the filter element and engages with a brine ring attached to the housing. The retaining ring bears against the upper end of the element as the housing is removed and forces the neck of the element to disengage from the end cap.
Although the use of the foregoing retaining ring arrangement has worked well, it still presents a number of drawbacks. First of all, the retaining ring, as a separate piece, requires separate additional assembly and disassembly steps and is subject to being misplaced and lost. Because the retaining ring is operatively attached to the housing, but not to the filter element, removal of the housing causes the retaining ring to bear on the end of the element which may cause damage to a fragile reverse osmosis membrane. Finally, in filter units where the brine ring must be welded to the interior wall of the housing, the need to provide a sonic weld relatively deep inside the housing has created problems of weld equipment accessibility and weld quality.
In accordance with one embodiment of the present invention, a filter cartridge retaining ring is secured directly to the neck of the product water tube that supports the membrane of an RO filter cartridge. The combined ring and cartridge are inserted together into an open-ended filter housing, and the housing is threadably attached to an enclosing end cap. Interengaging coupling parts on the retaining ring and the housing cooperate when the filter element is inserted into the housing to lock the cartridge therein. The end cap includes an integral center sleeve that surrounds the neck of the cartridge, when the housing is attached to the end cap, and provides therewith a liquid-tight seal. When the housing is subsequently removed from the end cap, as to change the filter cartridge, the filter cartridge neck is pulled from the end cap center sleeve because of the rigid interconnection between the cartridge and the housing provided by the retaining ring. This allows the housing and cartridge to be removed together.
In the preferred embodiment, a brine ring is attached to the inner wall of the housing adjacent the open end, and the coupling parts include interengaging connectors on the retaining ring and the brine ring. Preferably, the brine ring comprises a cylindrical tubular sleeve and the connectors comprise a pair of diametrically opposite slots in the upper end of the tubular sleeve and a pair of lugs on the retaining ring adapted to be received in the slots.
The reverse osmosis filter cartridge used in the filter assembly of this invention includes a cylindrical body that is covered with an impervious outer layer. A liquid-tight outer wrap seals the interface between the retaining ring and the outer layer of the cartridge body to provide an inner brine seal component. A first annular seal carried on an outer surface of the retaining ring engages an inner surface of the brine ring, upon insertion of the filter cartridge into the housing, to provide an intermediate brine seal component. A second annular seal is carried on an outer surface of the brine ring and engages an inner surface of the end cap, upon attachment of the housing to the end cap, to provide an outer brine seal component. In a preferred operational orientation, an untreated water inlet is located in the neck end of the RO cartridge, and the flow passage defined by the neck comprises a treated water outlet. The other end of the cartridge includes a brine water outlet. The three brine seal components form a composite brine seal that provides liquid separation between the brine water outlet and the untreated water inlet.
In an alternate embodiment of the brine ring, the ring may be made for a snap-in attachment to the cartridge housing, rather than welded or bonded to the inner wall of the housing. This modified brine ring is, however, intended to be left in place when the filter cartridge and retaining ring are removed for cartridge replacement.